Photovoltaic cells (“PVs”) are semiconductor devices that generate power by turning photons into charge carriers and electric current that can be harvested. Given the virtually unlimited source of solar energy, PVs are the most promising devices to generate truly clean and low-cost electricity with almost zero carbon emissions. To generate electricity from light, photons can be absorbed by a semiconductor material in a planar p-n junction device. An incident photon generates an electron-hole pair (“EHP”) in the conduction and the valance band of the semiconductor. The EHP is then separated by an electric field formed in a p-n junction. The electron is collected at one electrode and the hole is collected at another electrode, giving rise to an electric current in an external circuit connected to the electrodes, which can drive a load.
Adoption of solar technology for power generation has been very slow, however, due to cost and efficiency issues with current PVs. The majority of today's commercial solar panels for residential, industrial, or portable application are manufactured using single-crystal silicon (“Si”) wafers. State-of-the-art Si cells have a typical efficiency of about 18% to 20%. The solar power density is about 1 kW/m2 (AM1.5), requiring large panels to generate a modest power output. Given the cost of high purity (low defect density) Si material required for single-crystal cells, the overall high system cost prohibits it from being economically attractive to an average household or industry, even with current government incentives and subsidies. Either a significant increase in efficiency, a significant decrease in cost, or some degree of both at the same time, is needed in order to make solar technology truly competitive for power generation.
Solar cell efficiency is limited by two factors: Si intrinsic absorption and charge collection efficiency. First, the most common light absorbing material, Si, has a bandgap of 1.12 eV at room temperature. This bandgap makes it difficult to capture the long wavelength portion of the solar spectra associated with infrared light, which contains about 25% to 30% of the total solar incident energy. Furthermore, Si is an indirect bandgap material. The indirect bandgap reduces the absorption coefficient, α, such that relatively thick layers are needed to effectively absorb light. Usually, about 100 μm or thicker Si is needed. But, thicker Si translates directly into higher starting material cost and thus higher overall manufacturing cost. Shorter wavelength photons that have energies higher than the Si bandgap will lose their additional energy to heat (phonons) upon absorption. About 20% to 30% of the absorbed energy turns into heat, which is wasted. Today's solar cells use sophisticated light-trapping techniques to avoid light reflection from the surface of the cell. Such light-trapping techniques include surface texturing, antireflective (“AR”) coatings, and encapsulation in glass. Despite these techniques, about 10% of the incident light will still be reflected by the PV device and lost. As a result, about 60% of the energy is lost by the time the photons have been converted to EHPs.
Second, solar cell efficiency is limited by the charge collection efficiency. EHPs that are generated by light may nonetheless be lost during transport to electrodes. Some EHPs recombine along the way to the electrodes, turning useful energy into heat. Internal quantum efficiency (“ηint”) is a measure of efficiency in the charge collection process, which for state-of-the-art Si cells is about 90%.
These two factors, together with other non-idealities such as loss at the interfaces and area overhead in packaging, reduce the net system conversion efficiency to about 22% for the best single-crystal Si planar cells. Increasingly, obtaining increased efficiency by further optimization with Si is difficult and the gains are insignificant. Alternatively, utilizing other structures such as multi junction cells or alternative materials such as group III-V or II-VI semiconductors significantly increases cost and complexity of the PV. For example, multi-junction and gallium arsenide (“GaAs”) solar cells with efficiencies above 40% have been fabricated by several companies and are used for space and military applications. However, the high cost of these devices prohibits commercial use. It is clear that optimization of current planar Si PV technology alone cannot offer significant increase in efficiency or reduction in cost.
Another alternative technology that has emerged in the past few years is nanowire (“NW”) solar cells. A NW is a high aspect ratio wire with a diameter ranging from about tens of nanometers to several microns. NWs can be made tens of microns long. NWs can be synthesized, in single-crystal from, on a variety of low-cost substrates such as stainless steel, glass coated with indium tin oxide (“ITO”), or aluminum foil, using a low-cost chemical vapor deposition (“CVD”) or similar vapor-liquid-solid (“VLS”) processes. A significant amount of Si volume is saved by using an array of nanowires or similar high aspect ratio structures with gaps between them as opposed to a solid planar Si wafer. This approach lowers material costs significantly.
Although the total absorption volume is smaller compared to planar cells, NWs have been shown to have enhanced absorption. The absorption enhancement in the NW arrays is due to highly efficient light trapping. This light trapping has been shown to be more effective than texturing of a planar Si surface. See Garnett, E. and P. Yang, Light Trapping in Silicon Nanowire Solar Cells, 10 Nano Lett. 1082-87 (2010), which is fully incorporated herein by reference. Plus, there are enhancement effects stemming from the photonic crystal nature of the NW array. Plasmonic effects can also be used in order to guide and localize the field in the array and increase absorption coefficient α, thereby permitting a considerable reduction in the physical thickness of the absorber layer. Additionally, quantum confinement can be used in narrower wires to tune the bandgap for better absorption. NWs can also be decorated with quantum dots (“QD”) to further improve the spectral response. Given these advantages NW structures can compensate for the loss of volume compared to bulk planar structures and can reach total absorption figures comparable to those of the planar cells. See Garnett, E. C. et al, Nanowire Solar Cells, 41 Ann. Rev. Materials Res. 269-95 (2011), which is fully incorporated herein by reference.
NWs can also improve internal quantum efficiency. In traditional planar PV cells, the optical absorption path is parallel to the carrier transport path, putting absorption requirements at odds with the minority carrier diffusion length, LD. A thick layer of Si is needed for effective absorption, but most EHPs will recombine after traveling a diffusion length, LD, away from the junction before they are collected by the electrodes. The diffusion length, LD, depends on the doping level and purity level of the crystal and is typically several tens of microns for a high-purity and low-doped Si wafer. This is why most cells use a p-i-n junction, where an intrinsic region in between the p-region and n-region increases LD. Nevertheless, since LD is usually smaller than the cell thickness, some energy is lost to recombination in the depletion region, even with stringent cleaning protocols and low defect densities in manufacturing. In NW structures, the p-n or p-i-n junction can be fabricated in the radial direction, perpendicular to the NW axis along which incident light is absorbed. This will decouple the absorption and transport lengths. Carriers now only need to travel a distance equal to the NW radius to be collected and recombination is effectively eliminated. This type of structure can increase the internal quantum efficiency to close to 100%, which relaxes the purity/contamination requirements compared to planar PV cells because LD only has to exceed the NW radius. This reduces the cost. Another advantage of NW array structures is the relaxation of strain during growth, making it easier to fabricate hetero-structures and multi junction cells with materials other than Si on low cost substrates because the substrate does not need to be lattice-matched to the NW material. This makes it possible to lower the cost for non-Si based PV cells as well.
NW structures have shown great promise as low-cost PVs with overall efficiencies close to that of the state-of-the-art planar cells, and with possible quantum mechanical “boosters” to further improve absorption, paving a path for future scaling. However, great challenges have limited manufacturability and practical efficiencies of NW arrays. In particular, the NW PV needs to operate in radial junction mode to offer a reasonable efficiency. Forming a radial junction in a NW by post-growth doping is extremely difficult due to the small dimensions of the wire. The most common way to fabricate a p-n or p-i-n junction in a NW is to etch a p-doped or n-doped Si substrate and deposit a shell around the core, which is doped with the opposite-type impurity, either n- or p- respectively, during deposition. Deposition is usually a CVD process. In such a method the shell may not be fully crystallized, yielding many vacancy/dislocation defects. In addition, the interface between the shell/core or shell/i-region is often poor, causing massive recombination due to a high density of interface defects (Dit). The resulting PV cell has high leakage and recombination, resulting in a low open circuit voltage (“VOC”) and a poor fill factor (“FF”). For example, a VOC=0.5 V and FF=60% is among the best data published. The difficulties in controlling the radial junction and doping profile only become worse as the NW diameter is scaled down to take advantage of plasmonic and quantum effects. When the diameter of the NW is reduced to about 200 nm it may not be possible to create a junction at all, since the NW will become fully-depleted.
In order to compete with single-crystal planar PV cells, a radial p-n or p-i-n junction needs be created in each NW pillar which is very difficult to achieve and control and may be completely impractical in ultra-thin pillars. The proposed methods so far rely on etching doped Si to create the core of the pillar and a shell which is doped with the opposite type impurity during deposition by techniques such as CVD. This method leads to poor quality junctions with significant surface and interface recombination. The NW cells that have been fabricated in this fashion have had poor efficiencies so far making them unattractive for commercial applications.